"In safe doses the compound could improve their brains. It's possible it could delay the onset of Alzheimer's disease - but that's pure speculation.

"We hope to start clinical trials by the end of the year with a hundred - or even hundreds - of people."

Previous studies have suggested chemicals found in marijuana are effective against Alzheimer's disease.

In the latest experiments three groups of mice that were 18 (old), 12 (mature) and two months (young) were regularly injected with low doses of THC for 28 days.

The equivalent ages in human years would be 64, 58 and 20, respectively.

Both before and after the treatment they were challenged with a series of tasks that tested their learning and memory skills.

These included negotiating their way around a water maze, locating objects and recognising other mice.

In young mice THC impaired their performance - fitting in with research that has shown adolescents who smoke pot do worse at school.

But the same therapy actually improved learning and memory in both of the older sets of animals.

Prior to receiving THC they did poorly. But afterwards they were as efficient as the young mice had been before being given the compound that, interestingly, reduced their mental skills.

Prof Zimmer, a psychiatrist at Bonn University in Germany, said THC boosted genes that control neurons in the hippocampus - restoring them to the same patterns observed in young animals.

Psychoactive compounds found in marijuana - such as THC - exert their actions on the nervous system by interacting with the endocannabinoid system which deals with pain.

This is the brain's own internal version of cannabis chemicals which becomes desensitized as people get older. It has even been linked with beginning the process that leads to Alzheimer's.

The study published in Nature Medicine said THC can relieve age-related learning and memory problems - at least in mice.
Controversy

It said the potential use of cannabis to treat human neurological conditions such as epilepsy and multiple sclerosis - as well as pain - remains controversial.

Although marijuana use is generally believed to reduce mental function its effects on the ageing brain are not well-known.

Essential next steps include additional pre-clinical investigation to more thoroughly document the time course and persistence of these effects - as well as to determine their mechanistic basis.

Earlier this year a study by US scientists found THC destroys amyloid beta, the rogue brain protein that destroys neurons by causing inflammation. By exposing brain cells to THC the team stopped inflammation.

In the UK cannabis based drugs Nabilone and Sativex are licensed for use for nausea caused by chemotherapy and multiple sclerosis, respectively.